00-04、理解Servlet · 淺入淺出Java Web

本文依然使用[00-03、從JSP開始](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-03.md)中創建的項目HelloJSP。本文主要有以下內容： * 如何使用Servlet編寫Hello Servlet * 如何將Servlet與URL對應起來 * Servlet如何調用JSP * Servlet如何返回JSON數據 * 如何編寫一個Dispatcher ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#hello-servlet)Hello Servlet 項目結構如下： [![](https://github.com/someus/another-tutorial-about-java-web/raw/master/img/00-04/01.png)](https://github.com/someus/another-tutorial-about-java-web/blob/master/img/00-04/01.png) `HelloServlet.java`內容如下： ~~~ package me.letiantian.servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class HelloServlet extends HttpServlet { protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=UTF-8"); try (PrintWriter out = response.getWriter()) { out.println("<!DOCTYPE html>"); out.println("<html>"); out.println("<head>"); out.println("<title>Servlet HelloServlet</title>"); out.println("</head>"); out.println("<body>"); out.println("<h1>Servlet HelloServlet at " + request.getContextPath() + "</h1>"); out.println("</body>"); out.println("</html>"); } } @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } } ~~~ HTTP最常見的方法是GET和POST，在一個Servlet中對應的處理方法分別是doGet()和doPost()。`response.setContentType("text/html;charset=UTF-8");`?用來設置HTTP響應頭中的Content-Type。 PrintWriter對象out的輸出內容則是響應正文。 `web.xml`內容如下： ~~~ <?xml version="1.0" encoding="UTF-8"?> <web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"> <servlet> <servlet-name>HelloServlet</servlet-name> <servlet-class>me.letiantian.servlet.HelloServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/HelloServlet</url-pattern> </servlet-mapping> <session-config> <session-timeout> 30 </session-timeout> </session-config> </web-app> ~~~ 在這個配置中，`me.letiantian.servlet.HelloServlet`與URL`/HelloServlet`對應。?`session-timeout`設置了session的有效時間，單位是分鐘（不過目前的程序里還沒用過session）。瀏覽器訪問`http://127.0.0.1:8084/HelloJSP`會顯示404；訪問`http://127.0.0.1:8084/HelloJSP/HelloServlet`會顯示`Servlet HelloServlet at /HelloJSP`，這也正是`me.letiantian.servlet.HelloServlet`輸出的HTML的渲染結果。 ### [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#也可以使用注解將servlet和url對應起來)也可以使用注解將Servlet和URL對應起來首先清空web.xml中關于URL的配置，web.xml最終內容如下： ~~~ <?xml version="1.0" encoding="UTF-8"?> <web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"> <session-config> <session-timeout> 30 </session-timeout> </session-config> </web-app> ~~~ 然后對`me.letiantian.servlet.HelloServlet`類略做修改： ~~~ package me.letiantian.servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.annotation.WebServlet; @WebServlet("/HelloServlet") public class HelloServlet extends HttpServlet { // ...... } ~~~ 重新啟動項目，瀏覽器訪問效果和之前是相同的。 ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#servlet調用jsp)Servlet調用JSP 改寫me.letiantian.servlet.HelloServlet類，內容如下： ~~~ package me.letiantian.servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.RequestDispatcher; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.annotation.WebServlet; @WebServlet("/HelloServlet") public class HelloServlet extends HttpServlet { protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=UTF-8"); request.setAttribute("title", "Hello Servlet"); request.setAttribute("content", "你好"); RequestDispatcher rd = request.getRequestDispatcher("/WEB-INF/jsp/hello.jsp"); rd.forward(request, response); } @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } } ~~~ 在`WEB-INF/`下創建目錄`jsp`，然后在`jsp`目錄下新建`hello.jsp`，內容如下： ~~~ <!DOCTYPE html> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>${title}</title> </head> <body> <h1>${content}</h1> </body> </html> ~~~ 重啟該項目，訪問`http://127.0.0.1:8084/HelloJSP/HelloServlet`： ~~~ $ curl -i http://127.0.0.1:8084/HelloJSP/HelloServlet HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Set-Cookie: JSESSIONID=7CCCFD5467F8330066F827623802FB23; Path=/HelloJSP/; HttpOnly Content-Type: text/html;charset=UTF-8 Content-Length: 215 Date: Fri, 18 Sep 2015 08:09:58 GMT <!DOCTYPE html> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Hello Servlet</title> </head> <body> <h1>你好</h1> </body> </html> ~~~ ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#css等靜態文件放在什么地方)CSS等靜態文件放在什么地方在項目下建立static目錄，再這個目錄下添加`test.js`，內容如下： ~~~ console.log("hello world"); ~~~ 在`web.xml`添加以下內容： ~~~ <servlet-mapping> <servlet-name>default</servlet-name> <url-pattern>*.jpg</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>default</servlet-name> <url-pattern>*.png</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>default</servlet-name> <url-pattern>*.js</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>default</servlet-name> <url-pattern>*.css</url-pattern> </servlet-mapping> ~~~ 此時，項目結構如下： [![](https://github.com/someus/another-tutorial-about-java-web/raw/master/img/00-04/02.png)](https://github.com/someus/another-tutorial-about-java-web/blob/master/img/00-04/02.png) 啟動項目，訪問 ~~~ $ curl -i http://localhost:8084/HelloJSP/static/test.js HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Accept-Ranges: bytes ETag: W/"27-1442566151000" Last-Modified: Fri, 18 Sep 2015 08:49:11 GMT Content-Type: application/javascript Content-Length: 27 Date: Fri, 18 Sep 2015 08:58:00 GMT console.log("hello world"); ~~~ ~~~ $ curl -i http://localhost:8084/HelloJSP/static/test.js?time=123 HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Accept-Ranges: bytes ETag: W/"27-1442566151000" Last-Modified: Fri, 18 Sep 2015 08:49:11 GMT Content-Type: application/javascript Content-Length: 27 Date: Fri, 18 Sep 2015 08:58:09 GMT console.log("hello world"); ~~~ ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#servlet如何返回json數據)Servlet如何返回JSON數據將 ~~~ response.setContentType("text/html;charset=UTF-8"); ~~~ 修改為 ~~~ response.setContentType("application/json;charset=UTF-8"); ~~~ 。 `out.println`輸出JSON格式的字符串即可。 ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#編寫dispatcher)編寫Dispatcher 基于以上的學習，已經可以編寫一個分發器了。將HelloServlet.java修改為DispatcherServlet.java，內容修改為： ~~~ package me.letiantian.servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.annotation.WebServlet; @WebServlet("/") public class HelloServlet extends HttpServlet { protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/plain;charset=UTF-8"); try (PrintWriter out = response.getWriter()) { out.println("context: " + request.getContextPath()); out.println("request uri: " + request.getRequestURI()); out.println("params: " + request.getParameterMap()); } } @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } } ~~~ 運行項目，訪問結果如下： ~~~ $ curl http://localhost:8084/HelloJSP/user context: /HelloJSP request uri: /HelloJSP/user params: {} ~~~ ~~~ $ curl http://localhost:8084/HelloJSP/user?name=letian context: /HelloJSP request uri: /HelloJSP/user params: {name=[Ljava.lang.String;@49ea47b4} ~~~ ~~~ $ curl http://localhost:8084/HelloJSP/static/test.js console.log("hello world"); ~~~ （這個代碼并沒什么用～）從這段代碼中可以看到，我們可以通過request對象得到HTTP請求信息，特別是request URI。在這個程序的基礎上，我們可以繼續擴充它，使得其遇到某個URI，就調用指定的處理函數。慢慢地補充，一個框架就出來了。 ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#資料)資料本節中，JSP使用了表達式語言，可以參考： [JSTL 入門: 表達式語言](http://www.ibm.com/developerworks/cn/java/j-jstl0211/index.html) [JSP 表達式語言](http://www.runoob.com/jsp/jsp-expression-language.html)